Device and system for pressure measurement



April 16, 19.46.. c. A. GREEN DEVICE AND EYSTEM FOR PRESSURE MEASUREMENT Filed Jan. 11, 1944 V INVENTOR. CHRISTOPHER A. GREEN ATTORNEY Patented Apr. 16,1946

DEVICE AND SYSTEM FOR PRESSURE MEASUREMENT Christopher A. Green, University City, Mo., asaignor to Cnrtisl-Wrlght Corporation, a corporation of Delaware Application January 11, 1944, Serial No. 517,889

2 Claims. (Cl. 201-633) This invention relates to pressure indicating systems and is more particularly concerned with improvements in an extremely sensitive straintype pressure responsive unit for use in an indicating or recording system.

An important object is the provision of a sensitive pressure responsive unit which may experience a measurable amount of deformation due to pressure effects, and in which the deformation characteristics of the unit are physically related with or directly proportional to the existing pressure characteristics.

A further object is exemplified by the form and construction of the pressure responsive unit in which a pressure chamber or cell is defined, in part at least, by pressure sensitive or deformable walls whereby wall deformation, either in the expanding or contracting sense, may be utilized to produce a desired eflect in an indicating or recording system associated therewith.

Another object resides in the arrangement of the pressure sensitive unit for use with electrical type strain gauges and wherein the deformation of the pressure sensitive wall portions of the unit is employed to produce a change in the resistance value of the associated strain gauges and consequently a change in an initially balanced electrical bridge circuit, which is associated with the electric strain gauges.

An additional object of this invention is the provision of a compact yet sensitive pressure responsive unit, particularly adapted for use in restricted or closed locations, and one which may be made to give indications or pressure conditions through the interposition of an electrically responsive system including electric strain gauges in temperature compensating association with a balanced bridge system, together with indicating or recording instruments.

A more exact understanding of the invention may be had from the following detailed description and from the accompanying drawing, in which:

Fig. 1 illustrates one form and construction of the pressure sensitive imit associated with a simple electrical bridge circuit and visual indicator,

Fig. 2 illustrates a modified form of the pressure sensitive unit utilized in a more advanced electrical bridge circuit adapted for visual indication of pressure characteristics; and

- Fig. 3 is a furtherform,ofrpressuresensitive unit associated with j an electrical bridge circuit withparticularity to Fig. 1 thereof, the sensitive pressure responsive unit In is comprised of an outer body H of general thimble form, an inner thimble or body I! threadedly united with the outer body, and a closure cap is threaded onto the outer body. The cap, with the aid of a flat gasket or washer It, forms a pressure-tight seal for the adjacent ends of the associated thimble-like bodies. An annular chamber or space I! is provided between the outer and inner bodies, and an inlet port It formed in the base of the outer body II admits a medium under pressure to the chamber IS. The important structural characteristlc of the described unit I0 is exhibited by the circumferentially extending thin-wall portions i1 and ll of bodies H and 12 respectively and the provision of relatively thickened end portions for each body. As a result, pressure existing or permitted to exist in chamber It tends to expand the outer body element or member ii and compress the inner member l2. However the considerably thickened end portions of the unit resist most strongly the deformation due to this pressure while the more sensitive or thin-wall zones l1 and I8 experience substantially all the deformation. The principal axis of deformation extends circumferentially of the unit and particularly within the longitudinal extent of the walls l1 and It. It will be observed that the outer thin-wall ll experiences with positive pressures, for example, a tendency to expand or increase its circumferential dimension, while the inner thin-wall i8 undergoes a decrease in its circumferential dimension. Hence the sensitive walls undergo simultaneous deformation in opposite senses.

With a pressure responsive and sensitive unit of the type above described, the measurement of pressure in chamber I! then becomes a problem of measuring the extent of the body wall deformation in its principal or circumferential axis. It is important in the construction of the unit iii to choose suitable thickness dimensions for the thin-wall portions l1 and it, such that the resulting deformation shall have a direct or proportional relationwith the pressure existing within chamber It. It is also preferred that the material of which the unit is formed be charac- Y terizedas having a linear or sul'istantiall'yv linear stress-strain curve. Where thematerial fails to;. develop a lin'earstress-strain curve or lacks char- ;acteristics of that nature, a calibration curve may I be developed for a useful pressureran'geqjllor normal high :positive pressure *ranges, the 3 use or aluminum. steeL-or 'ailoys ofteacb ayfbj ring to the Wheatstone bridge.

found expedient. For example, a pressure unit constructedof commercial aluminum of the 248'! character has proved extremely practical for working pressures as high as 2500 to 3000 p. s. i. and as low as 500 p. s. i. For low pressure use, recourse may be had to a material possessing a deflection or deformation characteristic sufficient to permit a measurable change in circumferential dimension. In this later category, there is necessarily included plastics and phenolic resins generally.

A convenient and very accurate means by which measurement of the extent or degree of .wall deformation may be had involves the employment of commercially available resistance type electrical strain gauges arranged in accordance with the well-known Wheatstone bridge circuit,'and in which the strain gauges become one or more of the resistance legs of the bridge. The expression "bridge circuit" shall be understood as refer- The bridge circuits also include a source of direct current such as a dry cell or wet-plate battery and an indicator means of the galvanometer type. More specifically, a simple electrical indicating circuit arrangement, which may be useful with the pressure sensitive unit I! above described, includes constant resistances R1 and R: serially connected through a balancing potentiometer resistance element 20 as at 2| and 22 respectively. It is preferred that the fixed or constant resistance elements R1 and R: be substantially equal. The opposite side of the bridge circuit comprises an electrical strain gauge 23 associated with the internal surface of the inner thimble body I: and a second electrical strain gauge 24 associated with the external face of the outer thimble body II. The strain gauges 21 and 24 have a common connection at terminal 25 through conductors 26 and 21, respectively. Strain gauge 23 is also connected through conductor 28 with the terminal end 29 of resistance R1, while strain gauge 24 connects with the terminal end 80 by means of the conductor 3|. The current source or battery 32 is connected into the bridge circuits at terminal 25 and to the adjustable arm 33 of the balancing potentiometer. A suitable current flow meter or galvanometer ll is connected by conductors II and 38 to the bridge terminals 2! and 30 respectively. The galvanometer 34 functions in the well-known manner to indicate the direction of current flow in the bridge circuit.

The electrical strain gauges, in order that they will be accurate and representative of the actual wall strain due to pressure eifects, are carefully and completely bonded in any suitable manner to the surface of the wall so that they will be strained identically with the wall over the full range of pressures which may be encountered. The theory and application of resistance type strain gauges is well known and hence an extended discussion is deemed unnecessary. Suilice it to say that such gauges are characterized by an electrical conductor which is extensible and contractible and which, therefore, offers a resistance to the flow of an electric current which increases as the conductor is extended and decreases as the conductor is permitted to contract. In the present invention, it is proposed to take advantage of,the change of resistance characteristic of the strain gauge and to translate that condition into electrical values which have a direct or proportional relation with a pressure condition encountered or requiring observation and study.

assasva In operation, the invention contemplates that the improved and novel pressure sensitive or sensing unit ill will react to pressure conditions desired to be studied, whether such pressure be static, dynamic, or rapidly fluctuating in character, in a direct or proportional manner and that the strain produced in the sensitive wall portion of the unit shall be converted by the strain gauges directly to electrical units which may be indicated or recorded. To do this, the electrical type strain gauges are intimately associated with the strained surface and are most generally positioned with the major or principal axis thereof in coincidence with the principal axis of strain of the sensitive surface. The gauges are inserted in a bridge circuit of Wheatstone type and act to produce current flow conditions therein capable of being indicated or recorded. An important and advantageous characteristic of the pressure sensitive unit is its arrangement for automatically compensating for temperature conditions. The strain sensitive surfaces l1 and II necessarily undergo the same degree of expansion or contraction with changes of temperature and hence the associated strain gauges are aifected in the same degree and to the same extent. Accordingly temperature conditions have no serious or appreciable effect on the electrical measuring system.

A modified pressure sensitive unit 40 and electrical circuit is illustrated in Fig. 2. However, the theory of its use and operation is not different from that already set forth in connection with the arrangement of Fig. l. The principal structural differences reside in the method of eflecting a sealed joint between the inner and outer thimble members ll and 42. As shown the thlmble.

members 4| and II are threadedly assembled and the resulting Joint is then welded closed as at I. This results in a permanently sealed structure and one well adapted for use in high pressure work. The thimbles II and 42 are arranged in internally spaced relation, thereby providing an annular and cup-shaped chamber N. This chamber is placed in communication with a source of pressure throughgport 4| provided in the thickened base of the outer member 42. The pressure sensitive wall portions ll and I! or the outer and inner body members 42 and ll, respectively, are circumferentially stressed in the same manner as the corresponding sensitive wall pertions of the previously described pressure unit II.

The electrical indicating system associated with the pressure unit 40 comprises a Wheatstone bridge circuit in which strain gauges constitute the four arms or branches of the bridge. For example, strain gauges 48 and ll, securely cemented or bonded to the interior face of the cupped member 4| and arranged in a circum-" ferential position, are connected acrou bridge terminals Eli-ll and 52-43, respectively, by means of two strand conductors l4 and II respectively. Similarly, strain gauges I. and I1 circumferentially arranged and cemented to the outer surface of the cupped member 42, as indicated, are electrically connected respectively across bridge terminals 50-" and llll by two strand conductors 5! and II respectively. A balancing type potentiometer element II is inserted between strain gauge legs ll and I! and the movable contact arm 02 of this potentiometer is connected to one lead 83, the latter lead being connected through the battery I to the bridge terminal 50. The indicating galvanometer II is asaasra 3 connected in the bridge circuit between terminals Iii-l2 as shown.

In operation theuse of four strain gauges, two A on theinterior face of the unit and two on the exterior face. is found to be of considerable advantage in that there results greater current new conditions and better temperature compensation. This advantage is most useful for oscillograph or recording instruments where the .increased current flow is important for enhanced signal output. Moreover, the use of four strain gauges is advantageous when it is considered that a greater area of the sensitive wall portions of the pressure unit may be utilized for sensing and observing pressure conditions occurring within the chamber 44.

A further modification in the structural arrangement of the pressure sensitive unit is shown in connection with the improved circuit arrangement of Fig. 3. In this system the pressure sensitive unit 10 is essentially similar to the previously described units ill and I0 but differs in respect of the sealed joint between the outer and inner elements H and 11 respectively. In place of the gasket or washer element It or the welded seal 43, units l0 and 40 respectively, a jam fit seal is employed for the members II and 12 when assembled and this form of seal permits ready separation of the parts. One means of forming the Jam type seal is to provide each member with complementary beveled faces 13 and 14 which come into face-to-face contact when member 12 is threaded into the member H as shown. Suitable tool engaging faces or flats 15 are provided about the outer margins of each body member to facilitate assembly and disassembly of these parts of the unit Ill.

The pressure sensitive unit 10 is provided with a pair of interiorly positioned strain gauges IB'|1 which are cemented or bonded to the sensitive wall zone of the inner body member I2 so as to be strained directly with the circumferential wall strain. A second pair of strain gauges ll-I0 bonded to the exterior wall surface of the member II in the zone of the sensitive wall area are adapted to be strained along with the wall strain produced by a pressure condition within the chamber or space 80 of the unit 10. The chamber 80 is placed in direct communication with a pressurized conduit or body Bl or any other source of pressure or a space under pressure by a suitable connecting conduit 82. The several strain gauges 18, 11, I8 and 19 are connected in a Wheatstone bridge circuit at terminals 83, 84, l! and It in the usual manner so as to constitute the four arms of the bridge. A current source 81 connects into the bridge at terminals 83-" and the bridge output terminals are at It and 86. While a conventional galvanometer may be connected at the bridge output, it is preferred to incorporate an amplification unit, as at 88, and to insert an oscillograph or recording instrument, as at 89. The advantage of this latter arrangement lies in the fact that very small-current values in the bridge circuit may be picked up and amplified by the unit 88 for usein the instrument 89 selected or best adapted to the observations being made. Moreover, the current source I! may be either direct or alternating current as desired, a'nd if alternating current the units "and I! must also be adapted-for'alternatlng current.

' n g'Ihe above-described'electricalcircuitarrange ment is intended .toillustrate the advantages and utility ofthesensitivc ipressure cell or when 75 combinedwith an qually sensitive indicating means,oranoscillographorrecordingt7peinstrument. 'Whilethe last-described unit II is showninccnnectionwithaconduitorbody-ll containing a medium under pressure. it must be understood that the pressureunit in any of its described forms or equivalents may be utilised to advantage therewith or in many other arrangements not specifically shown or mentioned. It is herein preferred to utilise the pressure sensitive cell orunit in conjunction with four electrical strain gauges, so that the greatest accuracy in observing pressure conditions and characteristic pressure frequencies or fiuctuatims may be obtained. However. the arrangement of Fig. l is regarded as entirely satisfactory for the more stmdpyle or academic pressure observation and s u The foregoing discussion of the pressure sensing means or hollow body having wall portions sensitive to pressure conditions existing or developed within the pressure chamber has covered its application where atmospheric pressure or substantially atmospheric pressure exists exteriorly thereof. Additional and equally advantageous, uses for this pressure sensing means and associated electrical indicating or recording system include the measurement of atmospheric pressures or hydrostatic pressures. These latter pressure conditions may be sensed if the chamber or cell is sealed or made pressure tight. The pressure sensing device then becomu a sensitive and accurate altimeter or baromctei. It is also recognized as possible to am: the strain gauges upon the internal wall surfaces of the pressure chamber and to lead the electrical conductors outwardly through the port provided and through a tube or other means. There is thus achieved a device which may be readily and easily inserted in a tank containing a medium under pressure. A series of these latter devices may be utilized to indicate or sample successive stages ofdepth or pressure strata. Moreover, it is entirely possible to use the present device as a temperature indicating instrument by the simple expedient of filling the body chamber with a fluid such as alcohol or mercury or other common temperature responsive medium.

It should be understood that the present invention may be used wherever a positive or negative pressure indication or recording is desired and is not limited to the cases where a medium under pressure must be communicated to the interior of the chambered unit. Moreover it is recognized that certain changes and minor modifications may be made or suggest themselves but without departing from the spirit and intended scope of the invention as defined by the appended claims.

What is claimed is:

1. A pressure sensitive means adapted for use with indicating or recording instruments, the pressure sensitive means comprising a body member formed of an outer cylindrical shell structure and an inner cylindrical shell structure arranged to define a cellular space of generally annular extent, one of said shell structures having a port provided therein for the introduction to said cellular space of a medium under pressure, and a pressure. seal for the shell structures, said shell structuresfurther having wallportions capable of being deformed by pressure forces acting in a radial direction whereby the circumference of 'each said structure in the zone of the; deformable wall portions may be measurably varied in prop rtionto. in saidicellulan space and acting upon the deiorrnable wall portlons of said shell structures.

2. A pressure sensitive means adapted for use with indicating or recording instruments, the pressure sensitive means comprising a body member formed of an outer cylindrical shell structure and an inner cylindrical shell structure arranged to define a cellular space of generally annular extent, one or said shell structures having a port provided therein for the introduction to said cellular spaoe of a medium under pressure, and a pressure seal for the shell structures, said shell structures further having wall portions capable of being deformed by pressure forces acting in a radial direction whereby the circumference of each said structure in the zone of the deformable wall portions may be measurably varied in proportion to the pressure existing in said cellular space and acting upon the deformable wall portions of said shell structures, and a pair of electrical resistance type strain gages adhered one to the exterior of the deformable wall portion of said outer shell structure and the other to the interior of the deformable wall portion of said inner shell structure.

CHRISTOPHER A. GREEN. 

